# Batch Loader - Active Record # [![Build Status](https://travis-ci.org/mathieul/batch-loader-active-record.svg?branch=master)](https://travis-ci.org/mathieul/batch-loader-active-record) [![Gem Version](https://badge.fury.io/rb/batch-loader-active-record.svg)](https://badge.fury.io/rb/batch-loader-active-record) This gem allows to leverage the awesome [batch-loader gem](https://github.com/exAspArk/batch-loader) to generate lazy Active Record relationships without any boilerplate. It is not intended to be used for all associations though, but only where necessary. It should be used as a complement to vanilla batch loaders written directly using [batch-loader gem](https://github.com/exAspArk/batch-loader). **This gem is in active deployment and is likely not yet ready to be used on production.** ## Description This is a very simple gem which is basically a mixin containg replacement macros for the 3 active record association macros (refer to the [CHANGELOG](https://github.com/mathieul/batch-loader-active-record/blob/master/CHANGELOG.md) to know what is supported and what is not): * `belongs_to_lazy` * `has_one_lazy` * `has_many_lazy` Those are intended to use in replacement of the original Active Record class methods when you also want to generate a method to avoid N+1 calls to the database when accessed for the elements of a collection. For more details on why N+1 queries are common when fetching associations read [the batch-loader gem README](https://github.com/exAspArk/batch-loader/#why). For example let's imagine a post which can have many comments: ```ruby class Post < ActiveRecord::Base has_many_lazy :comments end class Comment < ActiveRecord::Base belongs_to :post end ``` Now we get a list of post objects and we want to fetch all the comments for each post. When we know in advance that we'll need the post comments, then Active Record query [#includes](http://api.rubyonrails.org/classes/ActiveRecord/QueryMethods.html#method-i-includes) will trigger a single query to fetch posts and comments. But often we don't know in advance in the code responsible to fetch the posts if we'll need access to the comments as well. When implenting a GraphQL API for instance, the post resolver doesn't know if the comments are also part of the GraphQL query.Using `#includes` in this case would be wasteful and slower for the cases when we don't need the comments. When using the lazy association accessor (i.e.: `post.comments_lazy`), a Batch Loader object is returned instead of a model relation and the query with the post id is buffered temporarily in the thread hash. No query to the database is executed yet. Calling the same association accessor on another post instance will add this post id to the list in the tread context. And so on until we access one of those Batch Loader objects returned. Only then is the database query executed and all Batch Loader objects are replaced by the records just fetched (not really replaced, they use delegation under the cover). It is important to note that Active Record association accessors return relations which can be chained using the Active Record query API. But the lazy association accessors generated by `batch-loader-active-record` return (for all intents and purposes) an active record instance or an array of active record instances which can't be chained. To benefit from the query batching we must first collect the lazy associations for each model instance in our collection, and only then we can start using them to access their content. Accessing a lazy object too early triggers the database query too early. For instance using `#flat_map` to collect and use the lazy objects would fail as `#flat_map` does access each element of the collection immediately in order to flatten the result. ## Installation Add this line to your application's Gemfile: ```ruby gem 'batch-loader-active-record' ``` And then execute: $ bundle Or install it yourself as: $ gem install batch-loader-active-record ## Usage Include the `BatchLoaderActiveRecord` module at the beginning of the model classes where lazy associations are needed, and use one of the lazy class macros to declare all lazy associations. ### Belongs To ### Consider the following data model: ```ruby class Post < ActiveRecord::Base has_many :comments end class Comment < ActiveRecord::Base include BatchLoaderActiveRecord belongs_to_lazy :post end ``` We need to know the `post` owning each instance of `comments`: ```ruby posts = comments.map(&:post_lazy) # no DB query executed yet posts.map(&:author_first_name) # DB query was executed # => ["Jane", "Anne", ...] ``` ### Has One ### Consider the following data model: ```ruby class Account < ActiveRecord::Base include BatchLoaderActiveRecord has_one_lazy :affiliate end class Affiliate < ActiveRecord::Base belongs_to :account end ``` Fetching all affiliates for the accounts who do have one affiliate: ```ruby affiliates = accounts.map(&:affiliate_lazy) # no DB query executed yet affiliates.first.name # DB query was executed affiliates.compact # => [#, #] ``` ### Has Many ### Consider the following data model: ```ruby class Contact < ActiveRecord::Base include BatchLoaderActiveRecord has_many_lazy :phone_numbers end class PhoneNumber < ActiveRecord::Base belongs_to :contact scope :active, -> { where(enabled: true) } end ``` This time we want the list of phone numbers for a collection of contacts. ```ruby contacts.map(&:phone_numbers_lazy).flatten ``` It is also possible to apply scopes and conditions to a lazy has_many association. For instance if we want to only fetch active phone numbers in the example above, you would specify the scope like so: ```ruby contacts.map { |contact| contact.phone_numbers_lazy(PhoneNumber.active) }.flatten ``` ### Has Many :through ### Consider the following data model with a has-many association going through another has-many-through association. Agents can have many phones they use to call providers: ```ruby class Agent < ActiveRecord::Base include BatchLoaderActiveRecord has_many :phones has_many_lazy :providers, through: :phones end class Phone < ActiveRecord::Base belongs_to :agent has_many :calls has_many :providers, through: :calls end class Call < ActiveRecord::Base belongs_to :provider belongs_to :phone end class Provider < ActiveRecord::Base has_many :calls end ``` We want to fetch the list of providers who were called by a list of agents: ```ruby agents.map(&:providers_lazy).uniq ``` This would trigger this query for the collection of agents with ids 4212, 265 and 2309: ```sql SELECT providers.*, agents.ID AS _instance_id FROM providers INNER JOIN calls ON calls.provider_id = providers.ID INNER JOIN phones ON phones.ID = calls.phone_id INNER JOIN agents ON agents.ID = phones.agent_id WHERE (agents. ID IN(4212, 265, 2309)) ``` ## Development After checking out the repo, run `bin/setup` to install dependencies. Then, run `rake spec` to run the tests. You can also run `bin/console` for an interactive prompt that will allow you to experiment. To install this gem onto your local machine, run `bundle exec rake install`. To release a new version, update the version number in `version.rb`, and then run `bundle exec rake release`, which will create a git tag for the version, push git commits and tags, and push the `.gem` file to [rubygems.org](https://rubygems.org). ## Contributing Bug reports and pull requests are welcome on GitHub at https://github.com/mathieul/batch-loader-active-record. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the [Contributor Covenant](http://contributor-covenant.org) code of conduct. ## License The gem is available as open source under the terms of the [MIT License](https://opensource.org/licenses/MIT). ## Code of Conduct Everyone interacting in the BatchLoaderActiveRecord project’s codebases, issue trackers, chat rooms and mailing lists is expected to follow the [code of conduct](https://github.com/mathieul/batch-loader-active-record/blob/master/CODE_OF_CONDUCT.md).